(1) Field of the Invention
The present invention relates to a communication device and a communication system that performs communication via a wireless communication medium. The present invention especially relates to a mobile communication device installed to/in a mobile object like an automobile or carried with a user, and to a communication system equipped with the communication device.
(2) Description of the Related Art
In recent years, an automobile telephone and a portable telephone that perform communication via a wireless communication medium, and a communication device such as a hands-free device that enables a user to perform communication without holding the device in his hand, have become popular.
This type of the communication device is movable with an automobile if it is installed to/in the automobile, or is movable with a user if it is carried by the user. Because of this, if the automobile or the user moves to an area where a radio wave, as a wireless communication medium, does not easily reach, i.e. an area in a poor communication status, it becomes impossible to perform communication with a device such as another telephone, and a telephone call may suddenly be disconnected. For example, since the communication status is not good in a tunnel or in a mountainous area, any communication becomes impossible for the communication device installed to/in the automobile when the automobile drives through such areas.
Therefore, a communication device which informs of the fact when approaching an area in the poor communication status has already been proposed as a conventional invention. (For example, see Japanese Laid-Open Patent Application No. 4-286430 (Patent Document 1).)
FIG. 1 shows a structure diagram of a communication device (an automobile telephone) in the above Patent Document 1.
As shown in FIG. 1, the communication device in the Patent Document 1 is a device installed in the automobile, and includes following elements: a geomagnetic sensor 901 that detects a driving direction of the automobile; a speed sensor 902 that detects a driving speed of the automobile; a map data memory unit 903 that memorizes a map; a distribution data memory unit 904 that memorizes a distribution map of a radio wave electric field intensity; a location unit 905 that detects a current location of the automobile, etc.; a telephone 906; a display unit 907 that displays characters and graphics; a speaker 908; an alarm lamp 909; and a main controller 910 that controls the telephone 906, the display unit 907 and the like.
In this communication device, the location unit 905 specifies the current position of the automobile on the map memorized in the map data memory unit 903 according to a detected result of a geomagnetic sensor 901 and the speed sensor 902, and predicts a time and a distance in which it is possible to perform communication through the telephone 906 with reference to the distribution map of the radio wave electric field intensity memorized in the distribution data memory unit 904. Then, the main controller 910 controls the display unit 907, the speaker 908 and the alarm lamp 909 based on the predicted result, and pre-informs the user that the automobile approaches an area in the bad communication status and that a phone call is going to be disconnected.
Also, similar to the communication device in the Patent Document 1, a communication device installed in the automobile has been provided which pre-alarms the user that the automobile approaches an area in the bad communication status and that a telephone call is going to be disconnected, specifies the current location of the automobile through GPS functions, and notifies a time and a distance in which it is possible for communication to be performed between the user and his calling party (for example, see the Japanese Laid-Open Patent Application No. 10-93497 (Patent Document 2).)
Additionally, there is a proposed communication device which suspends the user's outgoing call operation to prevent a sudden disconnection of a telephone call when the user approaches the area in the bad communication status (for example, see the Japanese Laid-Open Patent Application No. 10-313484 (Patent Document 3).)
The communication device (a hands-free device) of this Patent Document 3 is a device installed in/to the automobile, predicts whether or not the automobile reaches the area in the bad communication status after a specific time from the current location, and suspends an outgoing call if the automobile reaches the area. Then, the communication device makes the outgoing call, which had been suspended once as described above, when the communication device reaches an area in the good communication status. By doing so, the communication device prevents the sudden disconnection of telephone calls.
Although the communication devices in the Patent Document 1 and the Patent Document 2 pre-inform that the area in the bad communication status is approaching and that the telephone call is disconnected, the user needs to operate the telephone by himself to terminate the communication normally, which makes the user feel somewhat troubled.
Also, in the communication device in the Patent Document 3, a telephone call may suddenly be disconnected if a call is made and is connected to a telephone line but the communication device subsequently goes into an area in the bad communication status. Then, in the same way as in the communication devices in Patent Document 1 and the Patent Document 2, the communication device in the Patent Document 3 requires the user's operation to terminate the communication normally due to the deteriorating communication status, which makes the user feel annoyed.
Furthermore, if one of the devices in the Patent Document 1, Patent Document 2 and Patent Document 3 is installed in/to the automobile and operated by a driver, i.e. the user, he/she is required to drive the automobile while being annoyed by a troublesome operation, which may cause a car accident due to a lack of concentration on driving.